Protonation of Gly_n Homologues in Matrix-Assisted Laser Desorption Ionization

Abstract

Protonation reactions of Gly n homologues were studied in order to elucidate the effect of peptide chain length and matrix materials on the matrix-assisted laser desorption ionization (MALDI) mechanism. At 337 nm nitrogen laser excitation the relative ion yield-chain length relationship showed considerable variability in 3,5-dimethoxy-4-hydroxycinnamic acid (SA), 2,5-dihydroxybenzoic acid (DHB), 6-aza-2-thiothymine (ATT), and R-cyano-4-hydroxycinnamic acid (CHCA), the four matrices studied. For SA and ATT, a monotonic increase of relative ion yields (RIYs) was observed with increasing peptide length. Similar increasing pattern and significantly higher RIYs were found for the homologues of the Gly n series in the CHCA matrix with the exception of Gly. The particularly high MALDI ion yield of this amino acid in CHCA may be attributed to preferential embedding and/or more efficient condensed-phase proton transfer. To unveil the nature of the proton-transfer reactions in MALDI, the peptide length dependence of the measured RIYs was compared to the trends observed in proton affinities (PAs) of the same homologues as well as to ion-molecule reaction rates calculated using the Langevin cross section. Chain extension enhancements in MALDI RIYs of all the studied matrices cannot be explained either by the increase in ground-state ion-molecule reaction rates or by a linear dependence on gas-phase PAs. However, in SA and ATT positive correlation was observed between MALDI RIYs of Gly n homologues and their PAs. The RIYs observed in CHCA were significantly higher than in SA. This effect could be explained by the markedly lower PA of CHCA (183 ( 2 kcal/mol) compared to that of SA (204 ( 4 kcal/mol).

Secondary ion-molecule reactions in matrix-assisted laser desorption/ionization

“…15,16,18 -26 Also, there have been suggestions that ion-molecule reaction thermodynamics influence MALDI spectra in a systematic way. 9,16,23,26 -30 These studies were generally qualitative in nature, but point in the same direction as the model presented here.…”

“…3. Substance P can suppress sodiated valinomycin in the following manner [reaction (9)]: . MALDI mass spectra of valinomycin and of a mixture of valinomycin and substance P in DHB matrix (mole ratio 1 : 2.5 : 1000), illustrating the dissimilar analyte suppression effect.…”

Revisiting the Myths of Protein Interior: Studying Proteins with Mass-Fractal Hydrophobicity-Fractal and Polarizability-Fractal Dimensions

“…Relationship between mass, volume and polarizability of it isn't a simple one (an account of intricate dependencies of various parameters, in the context of protein electrostatics can be found from [46] ). FD-based schemes, owing to their ability to detect scale-invariance provide the template for an ideal integrative scheme that can connect atomic cloud dispositions to macroscopic polarizability.…”

Enolisation Kinetics of m-Nitro Acetophenone

“…Different catalysts (amino acids) show different values of dipole moments and are hence [ expected to show different catalytic effects [18][19][20][21][22]. Table 4 shows the effect of variation of the amino acid concentration on the rate of enolisation.…”

“…The greater is the dipole moment of the amino acid the greater is the rate. The values of the dipole movements are Lalanine = 6.4, Glycine = 13.3 D, DL-alanine = 14.2 D, and β-alanine = 17.4 D [18][19][20][21][22]. Zwitter ion is the active species which has been found instrumental in accelerating the rate of enolisation.…”